The European heavyweights face off on Wednesday in the first leg of the last-16 tie at the Bernabeu, with the Portugal international facing his former side for the first time since his £80 million move in 2009.

Despite the pressure of a world record fee, Ronaldo has netted a remarkable 182 goals in just 179 appearances for los Blancos, and Carrick has singled out the "special" forward as the main threat to United's European dreams.

"Everyone has fond memories of Cristiano [Ronaldo]," he told The Sun. "What he's done at Madrid is incredible and he's going to take some stopping.

"I think we will need two or maybe three to mark him to be honest, he's been that good for Madrid."

And while Barca star Lionel Messi has eclipsed Ronaldo on a personal level by winning the Ballon d'Or for the past four years, Carrick insists his former United team-mate's talent should be feared.

He continued: "[Ronaldo's] been sensational, to score so many goals so quickly - it's been astonishing.

"Because of Messi, he maybe hasn't had the pats on the back or the credit he deserves because the other guy has been winning everything there is to win but that shouldn't take away from what he has achieved at Madrid. He's a special player."